Polyurethane is well known as a base product for the manufacture of rigid foams which are used widely as insulators in refrigerators and freezers, in insulating panels for buildings, and for the refrigeration industry, in sheets for insulating transportation means, tanks, pipes, and even for the production of high-density rigid foams for furniture or for technical products in general.
Polyurethane is also well known as a base product for the manufacture of flexible or semiflexible foams which are used widely as cushions and mattresses in the furnishing industry, and as padding in seats, arm-rest, bumpers etc. in the car industry, where it is often used with the RIM-RRIM technology.
Polyurethane foam is produced by the intimate mixing and reaction of two liquid products, an isocyanate component, usually crude MDI, and a polyol component which contains all or some of the following products in a homogeneous mixture:
polyols--reactive products of suitable molecular weight for reacting with the isocyanate to form a rigid, flexible or semiflexible product: PA1 catalysts--generally tertiary amines or potassium, tin or lead salts, which regulate the reaction rate; PA1 surfactants--which affect the surface tension and regulate the formation of the foam; PA1 water which reacts with the isocyanate to produce carbon dioxide which acts as an expander; PA1 flame-retardants which regulate the behaviour of the foam with respect to fire; PA1 expanding agents, that is, low-boiling products which regulate the expansion of the foam; PA1 various additives such as fillers, dyes and pigments for various applications. PA1 continuous: the reactive polyurethane mixture is cast onto a conveyor belt, is restrained laterally, and is allowed to expand freely until the product has hardened; after curing, the product is trimmed and cut into sheets; PA1 discontinuous: the reactive mixture is poured into container tanks and is left to expand freely until a hard block of foam is formed; after curing it is trimmed and cut into sheets. PA1 continuous: the reactive mixture is cast onto various paper, metal or glass-cloth coverings where it expands to fit the coverings, forming a covered rigid foam panel; the panel is then trimmed and cut to size; PA1 discontinuous: the reactive mixture is cast into moulds or presses in which the coverings are positioned and is left to expand until it fills them completely, adhering to the coverings and thus creating covered panels. PA1 free fatty acids, (%): 0.50 (m.w.=280) PA1 amide, (%): 80.00 (minimum) PA1 appearance at 25.degree. C.: amber liquid PA1 ionic activity: non-ionic surfactant PA1 biodegradability: complete PA1 colour (Gardner): 4-9 PA1 free diethanolamine (%): 4.6-9.5 PA1 glycerine (%): 5.00 max PA1 pH (1% aqueous sol.): 8.5-9.5 PA1 flash point (.degree. C.):&gt;150 (Pensky Martens)
The polyol component and the isocyanate component are thermostatically controlled, metered, mixed and poured by means of suitable machines.
Various formulations of the polyol component are used to produce different types of rigid, flexible or semiflexible foams by various processes, some of which are described by way of example below.
Continuous and Discontinuous Production of Rigid or Flexible Foam Blocks
Continuous or Discontinuous Production of Rigid Roam Panels
Production of Refrigerators, Refrigerated Counters, Freezers
The reactive mixture is injected into moulds (forms) in which the refrigerator to be insulated is positioned. The expansion of the product insulates the structure of the refrigerator.
Production of Poured and/or Sprayed Rigid Foam
The reactive mixture can be sprayed or cast directly into the cavity to be insulated by means of suitable machines, creating an insulating filling or layer.
Production of High-Density Rigid Foams with Integral Skins (Simulated Wood)
High-density foams of 200-600 kg/cu.m. similar to wood are produced and are used mainly in the furniture industry, the reactive mixture being cast in moulds where it expands to fit the design of the mould.
Production of Flexible or Semi-Flexible Cold Moulded Foam
Low density flexible foam of 30-200 Kg/m.sup.3 and high density semiflexible foam of 200-600 Kg/m.sup.3 are produced and used mainly in the furniture industry and in the car industry, the reactive mixture being poured in moulds where it expands to fit the design of the mould.
Production of Unexpanded Products or Semi-Expanded Foam
Unexpanded final products are produced by simply casting the reactive mixture in a mould until the mould is completely filled.
Semi-expanded foam is produced by injecting the reactive mixture in the mould, where it expands to fit the design of the mould (RIM-RRIM Technology).
Polyurethane-based foams have been produced for a long time with the use, as expanding agents, of chlorofluorocarbons, known as CFCs, which cannot now be used since they are considered to be responsible for the hole in the ozone layer and hence to be ecologically harmful.
They have recently been replaced by hydrochlorofluoro-carbons, known as HCFCs which, though soluble in polyurethane systems as CFCs are and thus suitable for preparing a base which is very stable over time and suitable for being expanded to create foam, continue to present ecological problems, although less significant than those of CFCs, so that their use will be limited within a few years and, eventually, will no longer be permitted.
There is therefore the problem of finding an expanding agent which is ecologically permissible and can replace the known HCFCs in the preparation of rigid polyurethane-based foams.
The product should have a cost no greater than the cost of HCFCs, which is already twice the cost of CFCs, and thus risks putting insulating polyurethane foams out of the market.
Research has been directed for some time towards the use of pentane (normal-pentane, iso-pentane, cyclopentane), nC5, iC5 cycloC5, respectively, and mixtures thereof, but these have the serious disadvantage of very poor solubility in the source products of polyurethane foam, both in polyols and in isocyanates, and of being inflammable.
However, since various ways of controlling inflammability are known, the true obstacle to the use of pentane as an expander for polyurethanes is its insolubility or, in any case, its insufficient solubility in the polyol component.
C5 is particularly attractive as a product for replacing chlorofluorocarbons in the manufacture of expanded polyurethane, above all because of its cost which is about half that of the known CFCs.
However, its insolubility continues to be a serious obstacle to its industrial use, since the emulsions used at present, which result from the mixing of pentane with polyols and isocyanates, are difficult to control and have a high degree of instability, tending to separate even in quite short periods.
This tendency causes serious problems in normal industrial processes such as those, for example, for the insulation of refrigeration equipment since, every time the plant is stopped, for example, at weekends or during quite long breaks in operation, the mixtures/emulsions separate and, when operation starts again, it is necessary to carry out lengthy and laborious resetting.
It is generally necessary to empty the entire plant completely, to re-emulsify the components, to restart the plant and to carry out once more all of the adjustments necessary to produce the desired product.
The substitution of pentane for the known HCFC products in the manufacture of expanded polyurethane has thus given rise to a large number of attempts to produce extremely fine and stable emulsions of pentane in the polyols and isocyanates from which polyurethane is produced so that, in practice, they have the same behaviour as HCFC solutions.